Remove MASTER_SITE_AFTERSTEP from MASTER_SITES. The tarball on that server
does not pass checksum. The tigr.net copy still has the correct checksum,
so leave that in as the sole master_site.
All the files within each tarball had identical checksums.
While here, simplify do-install a bit.

Remove mastersites that are either not carrying the current version,
or are carrying a tarball that will not checksum.
PR: ports/67017, 67018
Submitted by: Roman Neuhauser <neuhauser at chello dot cz>
Approved by: maintainer timeout (4 weeks)